Voltage Drop Calculator

Enter your wire gauge, one-way circuit length, and current to find the voltage drop across the run - useful for checking whether a wire run meets standard 3% guidance for branch circuits.

Voltage Drop Calculator

Live

Wire gauge (AWG)

One-way length (ft)

Current (amps)

Source voltage

Conductor material

Voltage drop

2.96 V

Percent drop

2.47%

Within 3% guidance?

Yes

Assumes a single-phase circuit at 75°C conductor temperature.

The formula

Voltage drop = (2 x K x I x L) / CM K = 12.9 for copper, 21.2 for aluminum (ohm-cmil/ft at 75°C) I = current in amps, L = one-way length in feet, CM = wire's circular mil area

Example

12 AWG copper wire (6,530 circular mils), 50 ft one-way, 15A load, 120V circuit: drop = (2 x 12.9 x 15 x 50) / 6530 ≈ 2.96V, or 2.47% - within the commonly recommended 3% guidance for branch circuits.

Step-by-step guide

Select your wire gauge and confirm the conductor material.
Enter the one-way length of the run - not the round-trip distance, since the formula already accounts for both directions.
Enter the current the circuit will carry, and the source voltage.
Check the percent drop against your application's guidance - 3% is a commonly cited target for branch circuits, and 5% for the combined feeder and branch circuit.

Why voltage drop matters

Excessive voltage drop means equipment at the end of a long circuit run receives less voltage than intended, which can cause dimmer lights, motors that run hotter and less efficiently, or electronics that behave unpredictably. It's a bigger concern on longer runs and higher-current circuits, which is why detached garages, workshops, or long outdoor runs often need a larger wire gauge than the same load would require over a short distance.

Note: This calculator estimates voltage drop for planning purposes. Actual electrical installations must meet your local electrical code and should be sized, installed, and inspected by a qualified electrician.

Common mistakes

Entering the round-trip wire length instead of the one-way length - the formula's factor of 2 already accounts for the return path.

Using the circuit breaker's rating instead of the actual expected load current - voltage drop should be calculated for the real anticipated current draw, not the maximum breaker size.

Frequently asked questions

What voltage drop percentage is acceptable?

3% is a commonly cited recommendation for a branch circuit alone, and 5% for the combined feeder plus branch circuit, though this is guidance rather than a strict universal code requirement in every jurisdiction - check your local electrical code.

Why does aluminum wire need a bigger gauge than copper for the same job?

Aluminum has higher resistance than copper for the same cross-sectional area, so an aluminum wire needs to be larger (more circular mils) to carry the same current with an equivalent voltage drop.

Does temperature affect voltage drop?

Yes - resistance increases with temperature, so a wire running hot will have somewhat more voltage drop than the same wire at a cooler temperature. This calculator uses the standard 75°C reference figure used in most NEC-based calculations.

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